The present invention relates to a method of and an apparatus for thermal transfer recording, comprising transference to a printing sheet of an image-receiving layer of an intermediate transfer recording medium having an image recorded thereon by means of thermal transfer recording.
Heretofore, there have been proposed heat transfer printing processes which comprise the steps of bringing a thermal transfer printing sheet, comprising a coloring material layer provided at least on one surface of a substrate sheet, and an image-receiving material of an image-receiving layer of a sheet into pressure contact with each other between a heating device such as a thermal head and a platen roller, and transferring a coloring material contained in the coloring material layer of the thermal transfer printing sheet to the image-receiving material by heat which is selectively generated by the heating portions of the heating device according to image information for recording an image on the image-receiving material. Among these processes, a hot-melt transfer printing process and a heat sublimation transfer printing process have been widely employed.
The hot-melt transfer printing process is a method for forming an image in which use is made of a thermal transfer printing sheet obtained by providing, on a substrate sheet such as a plastic film, a hot-melt ink layer which contains a coloring material such as a pigment dispersed in a binder such as a hot-melt wax or resin; the coloring material is transferred, together with the binder, to an image-receiving material such as paper or a plastic sheet to form an image thereon by means of a heating device such as a thermal head to which energy is applied according to image information. The image formed in the hot-melt transfer printing process has a high density, and is excellent in sharpness. This printing process is therefore suitable for the recording of binary images such as letters and line drawings. Further, it is also possible to form a multi-colored or full-colored image by using thermal transfer printing sheets of yellow, magenta, cyan and black, placing the colors one over another on an image-receiving material.
The heat sublimation transfer printing process is a method for recording an image in which use is made of a thermal transfer printing sheet prepared by providing, on a substrate sheet such as a plastic film, a dye layer which contains a sublimable dye serving as a coloring material dissolved or dispersed in a binder resin, and an image-receiving material prepared by providing an image-receiving layer on a substrate sheet such as paper or a plastic sheet; the sublimable dye contained in the dye layer of the thermal transfer printing sheet is transferred to the image-receiving layer of the image-receiving material to form an image thereon by means of a heating device such as a thermal head to which energy is applied according to image information.
In the heat sublimation transfer printing process, the amount of a dye to be transferred can be controlled in every dot on a thermal head by the amount of energy applied to a thermal transfer printing sheet. For this reason, the gradation of an image can be reproduced by the gradual change of density. Further, since the coloring material used in this process is a dye, the image formed is transparent, and a half tone which is obtained by placing dyes of different colors one over another can also be excellently reproduced. Therefore, a high-quality full-colored image can be obtained by using thermal transfer printing sheets of three colors of yellow, magenta and cyan, or of four colors of the three colors and black, by placing the three or four colors one over another on an image-receiving paper sheet.
In the heat sublimation transfer printing process, it is required that the sheet to which an image is to be transfer-printed has a dyable image-receiving surface. Therefore, it was almost impossible to transfer-print an image on a sheet that does not have an image-receiving layer thereon. To solve this problem, a heat sublimation transfer method was proposed in Japanese Patent Laid-Open Publication No. 62-238791 published Oct. 19, 1987. According to this method, an image can be transfer-printed to an ordinary paper sheet without using a special sheet preliminarily provided with an image-receiving layer. This method comprises a step of forming an image in an image-receiving layer provided releasably on a substrate sheet of an intermediate transfer recording medium by using a thermal transfer sheet having coloring material layers thereon together with a thermal head, and a step of transferring the image-receiving layer having the image therein to the surface of a printing sheet which is an ordinary paper sheet. This known method therefore consists of an image forming step to the image receiving layer of the intermediate transfer recording medium and an image transfer step from the intermediate transfer recording medium to the ordinary paper sheet. These two steps can be carried out either in a line or in separate lines or off line.
When the above method is carried out in a line as disclosed in Japanese Patent Laid-Open Publication No. 61-106,273, drive systems for the two separate steps are made common to simplify the mechanism and synchronized to shorten the time of the operation. However, because there is a distance between an image forming section in which the coloring material is transferred and an image transfer section in which the image-receiving layer is transferred, it is difficult to exactly position an image forming area of the intermediate transfer recording medium in the image transfer section after the medium is fed from the image forming section so that the image transfer area tends to shift relative to the image forming area.
There is a further problem that the intermediate transfer recording medium cannot be fully utilized from the rear edge of an image forming area to the front edge of the succeeding image forming area so that there is a waste of the non-utilized portions of the transfer recording medium with resultant increase of the operational cost.
Moreover, in the case where the method is carried out off the line, it is impossible to determine the position of the image forming area when the image-receiving layer is to be transferred to an ordinary paper sheet, so that selective transfer of an image forming area having an image formed therein onto the printing sheet is difficult to perform.
Further, in the case of transfer-printing of color images, after the image forming of one color on the image-receiving layer, the image forming of the next color must be carried out to produce a superposed three- or four-color image in the image-receiving layer. In order to perform this superposed image forming, the intermediate transfer recording medium must be fed back to an initial position at which the image forming of the preceding color was started, for the image forming of the next color, This feeding back of the recording medium is difficult to perform with exact positioning. More specifically, it is difficult to feed back each image forming area for the next color printing to the exact position at which the forward edge of the image forming area is in exact registration with the forward edge position taken for the preceeding color image forming.
Further, when the known heat sublimation transfer method is carried out in a line as mentioned before, the intermediate transfer recording medium is fed concurrently in both the image forming section and the image transfer section, and this causes the following problems.
1. The pass length of the intermediate transfer recording medium is large so that it is difficult to control the tension appropriately with the result that non-uniformity of feed and wrinkling of the recording medium tend to occur. A thin film substrate is used for the recording medium. Therefore, if there is too large a tension, the film substrate will be stretched, causing inexact positioning, while if there is too small a tension, the film substrate will be fed obliquely and wrinkled.
2. If there occurs up and down movement of a thermal head or movement of a heating roll toward and away from a platen roll in one section of the printing apparatus during the transfer printing operation, the pass length of the intermediate transfer recording medium will change, causing variation in tension, and this influences the other section of the printing apparatus, causing non-uniform feeding and inexact positioning of the recording medium.
3. In order to perform at the same time the image forming to the recording medium and the image transfer to the printing sheet, the feed rate or speed of the recording medium must be made the same in both the image forming section and the image transfer section. This is rather difficult.
4. When it is required to change the feed rate of the recording medium in both the image forming section and the image transfer section, the image forming operation and the image transfer operation cannot be effected concurrently so that the operational efficiency will drop.
In order to transfer the image-receiving layer having the dye image formed thereon from the intermediate transfer recording medium to the printing sheet, a thermal head, a hot stamper, a heat roller or the like is ordinarily used as a heating means in the prior art.
In a method in which a thermal head is used to transfer an image-receiving layer to a printing sheet, pulses of repetitive heating and cooling are given to the thermal head. In addition, the thermal head is such that a large number of heating resistors are arranged on every picture element in the main-scanning direction, so that non-heating portions are produced between electrodes provided in the main-scanning direction, and it is thus difficult to uniformly heat the image-receiving layer. For these reasons, the image-receiving layer tends to have a crack after it is transferred to the printing sheet. When the image-receiving layer has a crack, oxygen can easily pass through it. Such a crack is therefore disadvantageous from the viewpoint of weather resistance, and becomes the cause of the deterioration of the durability of the image-receiving layer. In a method in which a hot stamper or a heat roller is used to transfer an image-receiving layer to a printing sheet, it is necessary, in order to attain quick printing response, to heat (preheat) the hot stamper or the heat roller even when printing is not conducted because the hot stamper or the heat roller takes time to reach a predetermined temperature. Therefore, not only the power consumption is large, but also there is the problem of safety. In addition, since the parts of the hot stamper or the heat roller are large, it is inevitable that the entire device becomes large.
Furthermore, it is difficult to control the energy which is applied to the image-receiving layer when it is transferred to the printing sheet. This is because when heat is excessively applied to the image-receiving layer, the surface thereof becomes rough, and when the application of heat is insufficient, the image-receiving layer cannot be fully adhered to the printing sheet.